1. Field of the Invention
This invention relates to a copying apparatus in which an original to be copied is illuminated by light source means whose light has a relative spectrum distribution variable with time.
2. Description of the Prior Art
There is known an apparatus in which, to prevent fluctuation with time of the quantity of incident light to a photosensitive medium, the light output of a light source, in other words, the emitted light from the light source, is received by a light detecting element and the electrical input to the light source is controlled so that the output signal of said element becomes constant, thereby making the light output of said light source constant. (See U.S. Pat. No. 4,124,294, for example.)
However, there the light source is an enveloped gas type light source like a fluorescent lamp, particularly, where the tube wall temperature during turn-on is low, not only the light output of the light source is varied with time but also the relative spectrum distribution of the light emitted from the light source is varied with time as will hereinafter be described in detail. This is because the rate of variation with time of the intensity of the emitted light of the enveloped gas differs from that of the intensity of the emitted light of the fluorescent material. When such a light source is used as an original illuminating light source, good image stability cannot be obtained in the known apparatus as described above. The reason is that depending on the relation among the spectrum sensitivities of the photosensitive medium and the light detecting element, the spectrum distribution of the light source, and the spectrum transmission factor and spectrum reflection factor of an optical system (lenses, mirrors, etc.) interposed between the light source and the photosensitive medium, the quantity of light to which the photosensitive medium responds is varied with time as will later be described, even if the light output received by the light detecting element as a quantity-of-light detecting element is constant. Accordingly, for example, where it is desired to produce a number of copies continuously, the quality of the copy image is gradually varied with lapse of time and, for example, even if the first copy is good in quality of image, copies produced after repeated cycles of copying may become inferior in quality of visible image and thus, the stability of the image may not be maintained.
The above phenomenon will be explained with respect to an actual example. As the original illuminating light source, use has been made of a fluorescent lamp whose relative spectrum distribution becomes such as shown in FIG. 1 of the accompanying drawings after a sufficient time lapse has occurred as required for the relative spectrum distribution variation of the emitted light to cease, for example, about five minutes, has elapsed from the turn-on thereof. The relative spectrum distribution of this fluorescent lamp immediately after the turn-on thereof is shown in FIG. 2 of the accompanying drawings. In FIGS. 1 and 2, the light in the wavelength range of 700-800 nm is chiefly the emitted light of the enveloped argon gas and the light in the wavelength range of 500-600 nm is chiefly the emitted light of the fluorescent material. The intensity of the emitted light of the argon gas decreases with time and the spectrum of the emitted light of the fluorescent material increases with time. Also, a silicon photocell has been used as the light source controlling light detecting element, and cadmium sulfide has been used as the electrophotographic photosensitive medium (photoconductive medium). FIG. 3 of the accompanying drawings shows the relative spectrum sensitivity a of the light detecting element and b of the photosensitive medium. A voltage has been applied to the photosensitive medium, and a current flowing through the photosensitive medium when light is applied thereto has been measured as the output. This output corresponds to the quantity of light to which the photosensitive medium responds. When the input to the fluorescent material has been controlled so that the output current of the light detecting element becomes constant, there has been obtained the result as shown in FIG. 4 of the accompanying drawings. In FIG. 4, c represents the output of the light detecting element which is constant while the output d of the photosensitive medium to which light is applied through an image forming optical system begins to increase immediately after the turn-on of the fluorescent lamp.
As is apparent from FIG. 4, by simply controlling the light source so as to render the output signal of the light detecting element constant, the quantity of incident light to the photosensitive medium may be made constant but the quantity of light to which the photosensitive medium responds cannot be made constant. This is because the relative distribution of the emitted light of the fluorescent lamp is varied with time as already described. As a result, the image density is varied with time and good image stability cannot be obtained.